I built a MPCNC and milled a few times with a Dremel, which worked well so far. Now I have changed to a spindle for more flexibility.
The motor Z-axis motor now has the problem that sometimes it does not manage to lift the gantry and skips steps. But this only happens in a very specific area when the spindle is lowered very far.
I checked the voltage of the driver (0.72V) and then raised it to 0.9V for a test. That improved the problem a bit, but it still occurs.
I also checked if there was anything in the way and the spindle was blocking, but I couldn’t see anything.
If the motor skips steps I can help a bit and push the gantry by hand, that releases it and the motor can lift it.
Another thing I noticed are strange noises that only occur when the gantry is in the area on the Z-axis where the problems occur.
I made a video showing the motor skipping steps and a video where i tried to capture the noises (normal motor sound in the beginning, then those “scratching” noises).
How can I attach/share the videos here? I tried to post a link but couldn’t save the topic with that link.
Do any of you have an idea what the problem might be?
Check the grub screws on the connector between the stepper and the lead screw. By hand (steppers off), twist the lead screw and run it up and down to see if it is binding anywhere. Check to make sure that the router is not touching the core at any point during the vertical travel. Even a close pass with the router can touch when vibration and deflection is taken into account.
For videos, usually people post on YouTube and then share a link. Perhaps since you are a first time poster, you cannot yet share links yet. You probably can put a link in slightly edited so that it is text and we can copy and paste the link.
thanks for the reply. I checked the grub screws, they are tight.
I moved the gantry up and down manually a few times and found nothing, not even a close pass between the gantry and the holding parts. But the lead screw was harder to turn in the area where the motor skips steps.
After that i removed the gantry and looked closely at the lead screw and found what i think is brass dust. Then i looked at the brass counterpart which holds the lead screw and found parts that looked kind of grinded. Here is a picture where i tried to capture this (i made more, but can only post one):
A little bit of (imperceptible) bend in the lead screw or misalignment can definitely lead to binding (at least on LowRider, which is what I’m familiar with). As you observed, your fingers will give you more feedback operating it by hand than any other way.
That looks pretty dry. Have you greased everything? I use white grease (lithium), but a dry graphite spray might be less messy.
Lowering your Z speeds could give you that extra bit or torque you need.
You should definitely lube those. They areeant to be lubed and the brass is meant to wear out first, and the nuts are sort of consumables. Though they should last a while.
Thank you for your answers. I greased the lead screw when i build the cnc, but i only had pretty thin machine oil and that didn’t seem to last that long. I will try the white grease you mentioned, hopefully this will last a bit longer (and maybe lube better).
I will also try to lower the z speeds, that sounds like a good thing to try.
But is there any “easy” way to check if something is misaligned in the z-axis? Although a bend seems more likely, since it happens only in a particular section of the lead screw…
